This invention relates to steering column assemblies and in particular to steering column assemblies that are adjustable for reach.
It is known to provide steering columns that adjust for reach. Typically, they comprise a two part steering shaft that can be adjusted in length by one part telescoping over the other. To hold the shaft at the desired length it is known to provide a telescopic steering column shroud that comprises a fixed shroud portion that is secured to a part of the vehicle and a moving shroud portion that can move relative to the fixed part to vary the overall length of the shroud. Bearings within the shroud portions support the steering shaft whilst allowing it to rotate. Typically the two portions comprise tubes, one tube fitting within the other as they are collapsed together.
In use a releasable clamp mechanism secures the moving portion in the desired position relative to the fixed portion. In the clamped position the clamp mechanism prevents accidental adjust to the reach of the steering. In an unclamped, adjustment, position, it allows the moving portion to move so that the reach can be adjusted.
The clamp mechanism typically acts between the moving shroud portion and a bracket that is fixed to a mount secure to the vehicle. The bracket, or at least part of it, could in fact be considered to form a part of the clamp mechanism. The shroud will therefore be fixed at two points, one connecting the fixed portion to the vehicle and the other connecting the moving portion to the vehicle through the bracket. A suitable clamp mechanism comprises a bracket having two dependent arms that are located to either side of a rail mounted on the moving portion. A clamp pin passes through an opening in one arm of the bracket, through openings in the rail and then through an opening in the other arm. One end of the clamp mechanism carries a fixed head and the other a cam that acts between a further fixed head and the arm of the bracket. The cam can be moved to squeeze the arms of the bracket onto the rail, clamping it in place. For reach adjustment the rail is provided with elongate slots through which the clamp pin passes. The length of the slot largely determines the limit of movement of the moving portion.
The bracket primarily locates the moving portion relative to the vehicle and the fixed portion when it is clamped. Most of the forces applied to the moving portion can therefore pass through the bracket to be reacted by the vehicle body. In the event of a crash, it is important that the moving portion can break free of the mount so that the moving portion is set free to telescope towards the fixed portion. This is typically achieved by providing one or more capsules between the bracket and the vehicle. In the event that a high force is applied to the moving portion, such as when a driver's torso strikes the steering wheel in a frontal impact, the capsules break allowing the bracket to break free of the vehicle. The moving portion is then free to collapse towards the fixed portion.
To aid in the absorption of the collapse force after the capsule has broken it is known to provide one or more energy absorbing devices that are deformed as the collapse continues beyond its normal range of adjustment. The deformation of the device absorbs the collapse energy in a controlled manner.
It is also desirable, but by no means essential, that clamping of the bracket to the rail may also simultaneously secure the moving portion to the fixed portion. The benefits of this would be to maximise the bending stiffness of the total column assembly in the clamped condition and to ensure that there is a predictable amount of sliding friction between the two portions, in the clamped condition, which can contribute a specified proportion of the total energy-absorbing crash force required during collapse.
A partial split in an outermost one of the two portions may be used to help ensure that the squeezing action of the cam-tensioned clamp bolt leads to effective Tube-to-Tube clamping simultaneously with the aforementioned clamping of the upper shroud portion to the said bracket.
As already stated the clamping mechanism in such a design, when not released, secures the upper portion to the bracket. The bracket is in turn rigidly attached to the vehicle by mechanically fusible connections (known as break Capsules) at all times, except in a crash. In a crash, the bracket moves slideably relative to the mount, the said capsules having been fractured by a proportion of the force of impact of the driver's torso on the steering wheel. The initial driver impact (or “breakaway”) force has to overcome a combination of the capsule fracturing force, the Tube-to-Tube friction and sliding friction between the bracket and the mount. The movement between the bracket and the mount can be optimised by providing a so called “Ride down mechanism” which acts between the mount and bracket to absorb energy.
Once the capsules have broken and the bracket has slid by a few millimetres relative to the mount, the ride down mechanism may come into play as a partial means of controlling the force required to continue the telescopic collapsing of the column to the full limit of its stroke. The ride down mechanism may comprise a strip of metal that is attached to the bracket at one end and is shaped with a loop in such a way that it is progressively deformed, absorbing energy, by being dragged over an abutment on the mount during the telescopic collapsing of the column. Typically, the loop of the Energy Strap will have some initial clearance relative to the said abutment so that it does not contribute to the initial breakaway force.
The force which, in a crash, is required to act on the steering wheel in order for the column to telescopically collapse is usually specified to be much greater than the force which would realistically be applied to the wheel by a driver either in normal driving or when adjusting the position of the steering wheel. It is considered that some drivers could, in extremis, exert a sustained forward axial force of up 1500N on the wheel. Alternative, a shock load of up to 2500N could be realised if a driver rapidly adjusts the position of the upper column such that it impacts abruptly on the forward limit stop of the Reach adjustment travel.
A typical specification for crash-collapse force (as would be given by a vehicle manufacturer) would require that an axial force of approximately 6000N should be required to cause initial disconnection (breakaway) of the bracket from the mount in a crash and that a force of 2000N to 4000N would be required to cause collapse through the remainder of the telescopic travel.
However, with the type of Tube-in-Tube reach-Adjustment column described above (i.e. one having a partially split outer tube or other device for ensuring effective Tube-to-Tube clamping), it should be noted that the resistance to column collapse in crash is partly provided by the tube-to-tube friction. Typically, this tube-to-tube friction would be approximately 1500N. It should be noted that the contribution from tube-to-tube friction is not present while the column is unclamped for the purpose of adjusting the position of the steering wheel. Nevertheless, the remaining 4500N (i.e. 6000-1500) of breakaway resistance still present is ample to resist the potential “abuse” loads that a driver could inflict. (Note that the remaining 4500N of breakaway force derives from the strength of the said Capsules and from friction between the bracket and the mount.)
A problem may arise, though, if a vehicle manufacture specifies an unusually low breakaway force threshold; e.g. 3000N (in the clamped condition). In this event, the remaining breakaway resistance still present when the column is unclamped will only be 1500N (i.e. 3000-1500) and this will be insufficient to ensure that the Capsules are not fractured, and the crash stroke is not initiated, by abuse loads deriving from the driver during adjustment of the steering wheel.
In such a prior art design it may therefore possible for a load to be applied by a driver during adjustment that is great enough to cause damage to the steering column assembly by causing the capsules to break and perhaps cause some initial movement between the bracket and the mount. This may comprise the safety of the steering assembly and adversely affect its performance in a crash.
In EP 2 022 699 A2 it is proposed to provide a means for bypassing the ride down mechanism when the clamp mechanism is unclamped. The solution described is to provide a load transfer means that takes the axial load from the upper shroud directly onto the lower shroud when the column reaches its shortest length, away from the ride down mechanism. Several arrangements are disclosed for bypassing the ride down mechanism. The applicant has appreciated that all the arrangements disclosed have significant limitation and has sought to provide an alternative solution that overcomes at least partially the limitations.